Carbon nanotubes (hereinafter also referred to as “CNTs”) are carbon structures in which a carbon sheet (i.e. a sheet of graphite) constituted by carbon atoms planarly arranged in a hexagonal pattern is cylindrically closed. These CNTs are categorized into multi-walled and single-walled CNTs. Single-walled CNTs (hereinafter also referred to as “SWCNTs”) are known to have such electronic properties as to exhibit metallic properties or semiconducting properties depending on the way in which they are wound (i.e. the diameter and the degree of helicity).
Semiconductor SWCNTs, which absorb and emit light in a near-infrared region (800 to 2000 nm) where biopenetrability is high, are expected to be extremely useful as fluorescent probes that detect the functions of cells and organisms. Among them, a wavelength region of 1200 to 1400 nm is a region where biopenetrability is highest.
By introducing oxygen atoms or functional groups into these semiconductor SWCNTs, a change in emission wavelength can be effected. For example, irradiating, with light, a mixture of an aqueous solution containing SWCNTs dispersed by a surfactant and water containing ozone added thereto causes a chemical reaction whereby oxygen atoms partially substitute for carbon in the nanotube walls (Ghosh et al., Science, 330, 1656-1659 (2010) and Miyauchi et al., Nat. Photonics, 7, 715-719 (2013)). In a case where oxygen atoms are introduced in this way, most of the oxygen atoms form ether bonds with the SWCNT walls, with the result that the SWCNTs become lower in emission energy than they originally were by approximately 150 meV. Such a chemical modification also has the advantage of increasing the luminescent quantum yield of the SWCNTs.
In addition, by covalently introducing functional groups into semiconductor SWCNTs by an organic synthesis technique, the emission energy can be reduced by approximately 160 to 260 meV (Piao et al., Nat. Chem., 5, 840-845 (2013), Zhang et al., JPCL, 4, 826-830 (2013), and Brozena et al., ACS Nano, 8, 4239-4247 (2014)). For example, in the case of binding of hexanoic acid, the emission energy of the SWCNTs undergoes a low energy shift of 260 meV, and the emission of light from the SWCNTs is believed to be attributed to trion generation (Brozena et al., ACS Nano, 8, 4239-4247 (2014)).